This invention relates generally to the sterilization of medical and surgical products, and more specifically to a test pack adapted to be processed through a conventional sterilization cycle in a prevacuum steam sterilizer to check the proper operation of the sterilizer.
In the sterilization of medical dressings and instruments by steam, such dressings or instruments are conventionally assembled in bundles or packs and placed in a steam sterilization chamber which is sequenced through an appropriate sterilization cycle. Individual packages may contain stacks of towels, dressings, sponges and similar materials or may contain desired complements of surgical instruments or equipment for specific surgical procedures.
A current method of steam sterilization practice involves the placement of such packs in a sterilizer, the evacuation of air from the sterilizer and the introduction of saturated steam at a desired pressure to produce a desired temperature (typically 270.degree.-276.degree. F.) for a selected period of time. Prevacuum sterilizers operating in this manner have frequently replaced the downward displacement or gravity air discharge sterilizers previously used. The significant advantage of the prevacuum method is that removal of air before introduction of steam permits the rapid penetration of steam throughout the surgical pack. Elimination of air is necessary in all steam sterilizers since air trapped in the packages would prevent sterilization of the portion of the pack's interior where it collected. With the prevacuum method, the time required for steam penetration in a typical sterilization cycle is greatly reduced, and prevacuum sterilizers currently operate on a relative short cycle (with an exposure time on the order of 4 minutes at 134.degree. C.). The operation of such sterilizers is well described in John J. Perkins, Principles and Methods of Sterilization, in the Health Sciences, published by Charles C Thomas, Springfield, Illinois, Chapter VI, "Prevacuum High Temperature Sterilization."
The ability to sterilize instruments in a shortened time, however, is dependent on the assumption that air is properly evacuated from the sterilizer. This includes the expectation that the vacuum system is functioning properly to evacuate the chamber initially and that there are no air leaks in the sterilizer or the vacuum lines which would permit introduction of air after the vacuum is drawn. During routine use, wear on certain sterilizer parts will eventually result in air leaks, and there is a need to test the sterilizer on a periodic (daily) basis to ascertain whether the vacuum system is functioning properly.
In 1961, a test procedure was proposed by J. Dick et al. and described by J.H. Bowie, et al. of the Department of Microbiology, Royal Infirmary, Edinburgh, Scotland in an article appearing in The Lancet, Mar. 16, 1963, pp. 586-587, which suggested a protocol for determining that the sterilizer was in proper working order, and that the vacuum system was operating properly. As indicated in the article, residual air in the system at the time steam is introduced will be swept by the steam pressure into the pack, usually to the pack center. Trapped air in the pack inhibits proper steam penetration.
According to the Bowie and Dick protocol, autoclave tape was used in combination with a stack of surgical towels to test the working order of the sterilizer on a daily basis. Autoclave tape is an adhesive tape having printed on its top surface stripes of a chemical composition which has the property of changing color, for example from white to black, upon exposure to steam at an appropriate temperature for an appropriate period of time. According to the Bowie and Dick protocol, such tape was placed on a fabric sheet in a cross configuration, and the fabric sheet was then placed within a stack of folded surgical towels. The entire assembly was placed within the sterilizer. The sterilizer was run through its usual cycle with an exposure time of three and one-half minutes at 133.degree.-134.degree. C., after which the tape cross was examined to determine whether steam had completely penetrated the towel stacks effectively. A uniform color change was indication of a pass, and the presence of incompletely changed indicator color was a failure.
This protocol is currently in widespread use and is described in the Association For The Advancement Of Medical Instrumentation (AAMI) Recommended Practice: Good Hospital Practice for Steam Sterilization and Sterility Assurance, 1988 ed., published by the AAMI, 1901 North Ft. Myer Drive, Suite 602, Arlington, VA 22209. As indicated in Paragraph 6.7 of that publication, entitled "Prevacuum Sterilizer Residual Air Test," the test involves the use of 100% cotton huckaback towels, freshly laundered but not ironed (in view of the fact that excess dryness may affect the test results) folded in a 9".times.12" configuration and piled 10"-11" high. The details of the Bowie and Dick procedure are described in the above-referenced Recommended Practice. Standards for sterilizer performance in other parts of the world may require different pack constructions to test for the desired sterilizer efficiency. For example, test packs which would provide an acceptable challenge in the U.S.A. would be considered to be inadequate in Europe, whereas a test pack considered to be adequate in Europe would overchallenge U.S. sterilizers.
The testing of prevacuum sterilizers according to the Bowie and Dick protocol involves a number of important shortcomings. Firstly, the test is subject to individual execution by the sterilization section of the hospital on a daily basis, and the various requirements of the Bowie and Dick Protocol--namely the type of towels or other fabrics used, their condition, age and the like (all of which affect the significance of the test result)--may vary widely from day to day and from institution to institution. Secondly, the performance of the Bowie and Dick protocol is relatively inconvenient and costly in that the expense of laundering towels (which cannot be thereafter used without relaundering), assembling the test arrangement and the like involve costly hospital labor. Additionally, certain hospitals have elected to eliminate laundry facilities entirely, utilizing only single-use disposable fabrics for their procedures, making the proper conduct of the Bowie and Dick protocol more inconvenient.
Attempts have been made to permit the use of a test sheet without the use of the conventional stacks of cotton towels in accordance with the Bowie and Dick protocol. As indicated in the literature on the Bowie and Dick protocol, various types of defects are most frequently found in sterilization equipment. Principal among these are (1) inadequacy of initial vacuum, leaving residual air within the packs, and (2) air leaks within the chamber or vacuum system which permit the re-entrainment of air after a vacuum has been drawn. In designing a test pack to evaluate both these types of flaws, as well as others, it was discovered that there are a variety of different constraints operating.
For example, to the extent a flaw is present in the vacuum system which prevents a sufficient vacuum from being formed in the first instance, a test pack having a low porosity and/or high bulk is more likely to reveal a flaw of this type than one having a high porosity and/or lower bulk. As the vacuum is drawn, the air within the test pack tends to be drawn out of the pack. Low porosity and/or very bulky material surrounding the test sheet would tend to increase the difficulty of removal of such air and increase the likelihood that the test pack would indicate a flaw.
On the other hand, flaws caused by leaks involve the introduction of air into the sterilizer after the vacuum has been drawn. In this situation, the relationship between the porosity of the material surrounding the test sheet and sensitivity of the test is the reverse. The less porous and/or more bulky the material surrounding the test sheet, the less likely it is that air introduced into the sterilizer after the vacuum has been drawn (as in the case of a leak) will re-enter the test pack. Accordingly, with respect to a flaw resulting from air leaks after a sufficient vacuum has been drawn, low porosity material surrounding the test sheet would tend to make it more difficult for air from such a leak to enter the pack and decrease the likelihood that such a test pack would indicate a flaw.
In order to properly test the sterilizer, a test pack must provide an appropriate, but not excessive challenge to the vacuum system using as a guideline the challenge provided by the currently accepted towel pack standard for the Bowie and Dick protocol. It is desirable to design a relatively small, inexpensive and disposable test pack which achieves a challenge to the sterilizer comparable to the challenge provided by the accepted Bowie and Dick protocol.
Exemplary of the various attempts to provide a Bowie and Dick-type test pack without using towels are the multilayer test packs described in U.S. Pat. Nos. 4,486,387; 4,576,795; 4,579,715; 4,596,696; and 4,692,307. For example, according to U.S. Pat. No. 4,486,387, a multilayer disposable test pack is composed of a sterilization test sheet having defined areas adapted to change color in response to the presence of steam under selected exposure conditions, surrounded by a set of disposable non-woven porous sheets of material arranged in overlying relation above and below the test sheet. The innermost sheets of material form an inner core region around the test sheet, and the remaining sheets form an outer shell region, with the porosity and bulk of the inner core region and the porosity of the outer shell region being selected so as to define a desirable challenge to the sterilizer.
The test pack of U.S. Pat. No. 4,486,387 is designed to be used according to the accepted Bowie and Dick protocol and to be placed in an otherwise empty sterilization chamber, sequenced through a predetermined cycle and removed. The pack is then opened and the interior test sheet examined for evidence of inadequate steam penetration, air bubbles and like defects. The presence of such defects indicates faults in the vacuum or other systems within the sterilization unit which require evaluation and repair. The test is intended to be performed on a daily basis with the interior test sheet forming a permanent record of such testing. The non-woven sheet material and the remainder of the pack are disposed of after a single use.
Such a test pack provides detection of the common sterilizer flaws in a manner comparable to the Bowie and Dick Protocol. Nonetheless, these multilayer test packs have not been found to be entirely satisfactory. Their complex construction requires a large number of sheets to be arranged in appropriate sequence and number and then packaged with overwrap or placed in an outside container providing a predetermined tightness. The complexity of the construction and the care required in assembling the same adds to the cost of the product. While the conventional multilayer test packs are generally about 5.times.5.times.7/8 inches in the U.S. and about 5.times.4.times.1 inches in Europe, and thus substantially smaller than the towel packs required by the Bowie and Dick protocol, they are still relatively bulky when stored in quantity for daily tests. Accordingly, the need remains for a test pack which is composed of fewer sheets, thereby reducing the complexity of the construction and the care required in assembling the same, and thus the overall costs of the product. Further, the need remains for a test pack which is somewhat smaller than the presently available test packs, so that they present less bulk when stored in quantity for daily tests.
While some of the conventional multilayer test packs utilize a non-porous sheet of reduced steam and/or air permeability in order to enable a reduction of the overall size of the test pack without compromising the challenge presented to the stabilizer, the addition of the non-porous layer reduces the porosity of the test pack considerably and typically only one such sheet is used to either side of the test sheet (that is, one non-porous sheet intermediate the test sheet and each bundle end surface). It is in conventional thinking that the effect of a non-porous sheet is so strong that only one such non-porous sheet should be used between the test sheet and a bundle end surface or, if more than one is used, each non-porous sheet should be separated by numerous porous sheets.
Accordingly, it is an object of the present invention to provide a relatively small, inexpensive and disposable test pack for use in prevacuum steam sterilizers to determine whether the sterilizer is functioning in accordance with proper standards by simulating air evacuation and steam penetration conditions of the conventional pack described in the Bowie and Dick protocol so as to define an appropriate challenge for a residual air test in a prevacuum sterilizer.
Another object is to provide such a test pack for prevacuum sterilizers which provides a repeatable and consistent standard for testing the working order of the sterilizer from day to day and from sterilizer to sterilizer, yet requires neither assembly by hospital personnel nor the use of hospital linen or laundries for its initial use.
A further object is to provide such a test pack utilizing fewer layers than other comparable multilayer test packs.
A still further object is to provide such a test pack which is more compact than a conventional multilayer test pack offering a similar challenge in at least one and preferably two or even three dimensions.